1. Field of the Invention
The invention relates to a vane pump including a rotor having a plurality of slits housing vanes and extending in a radial manner, the vane pump configured to discharge a fluid, sucked into the vane pump through inlet ports, to outlet ports as the rotor rotates.
2. Description of the Related Art
Vane pumps have been used to, for example, actuate various kinds of hydraulically-driven machines. Such a vane pump includes a rotor holding a plurality of vanes. As the rotor rotates in a cam ring having an inner peripheral cam face, a hydraulic fluid sucked into the vane pump through an inlet port is discharged into an outlet port. The vanes are housed in slits formed in the rotor in a radial manner, and the vanes are movable in a radial direction of the rotor. The vanes define a plurality of pump chambers between an outer peripheral surface of the rotor and the inner peripheral cam face. In some of these vane pumps, a cam ring has an elliptically-shaped inner peripheral cam face, and a first inlet port, a second inlet port, a first outlet port, and a second outlet port are open into a rotor chamber in the cam ring (see, for example, Japanese Patent Application Publication No. 2001-27186 (JP 2001-27186 A)).
In such a vane pump having two outlets, as the rotor rotates together with the vanes, the hydraulic fluid sucked into the vane pump through the first inlet port is discharged into the first outlet port and the hydraulic fluid sucked into the vane pump through the second inlet port is discharged into the second outlet port. This configuration makes it possible to supply a low-pressure hydraulic fluid and a high-pressure hydraulic fluid to hydraulic-fluid supply destinations, by varying the throttling amount between a fluid passage for the hydraulic fluid discharged into the first outlet port and a fluid passage for the hydraulic fluid discharged into the second outlet port.
The vane pump described in JP 2001-27186 A has a first back-pressure groove and a second back-pressure groove that are communicated with deep portions of the slits (i.e., portions of the slits that are close to the center of the rotor). The first back-pressure groove and the second back-pressure grove are used to push the vanes out of the slits and to push distal ends of the vanes against the inner peripheral cam face. The first back-pressure groove is communicated with the slits that house the vanes defining pump chambers communicated with the first inlet port and the first outlet port. The second back-pressure groove is communicated with the slits that house the vanes defining pump chambers communicated with the second inlet port and the second outlet port. The pressure in the first outlet port is introduced into the first back-pressure groove. The pressure in the second outlet port is introduced into the second back-pressure groove.
When a vane pump having two outlet ports as described in JP 2001-27186 A is used, for example, to actuate an automatic transmission of an automobile, the rotor is rotationally driven by the engine torque transmitted via a torque converter. Thus, the axis of rotation of the rotor is disposed horizontally. In this case, the vanes positioned above the axis of rotation of the rotor may move into the deep portions of the slits under their own weight.
When some of the vanes move into the deep portions of the slits, no pressure is generated in one of the first outlet port and the second outlet port, which is positioned on the upper side, and thus no pressure is introduced into the back-pressure groove communicated with the outlet port positioned on the upper side. Therefore, until the rotation speed of the rotor becomes high enough to allow the vanes to be centrifugally pushed out of the slits, the pressure in the outlet port positioned on the upper side may not increase. Such a phenomenon is particularly noticeable when the oil viscosity is high, for example, when a vehicle is started at a significantly low temperature of −30° C. or lower.